1. Technical Field
The present invention is directed to a planar transformer and output inductor structure with a single planar winding board and two magnetic cores. This miniaturized component is used in power supplies and specifically in DC-to-DC converters.
2. Description of Related Art
To meet ever-increasing demand for high speed and miniaturization of digital devices, microelectronic circuits are using lower and lower voltage. 5 V and 12 V are no longer dominant power supplies used in microelectronic circuits. 3.3V, 2.5V, 2V, 1.8V, 1.5V, and even 1.2V are becoming standard voltage in many electronic devices. Actually, some next-generation high-speed microprocessors and DSPs need sub 1V as their supply voltage.
Migration to lower supply voltage and size miniaturization is rapidly changing power supply design and packaging technologies. The high switching frequencies together with soft switching and the synchronous rectification technologies help to reduce the losses and size of the power supplies dramatically.
On the other hand, as the power semiconductors and signal semiconductor devices are getting smaller and smaller, the size reduction of the power magnetic devices, which play critical roles in power supplies, becomes more and more crucial. The use of planar magnetics helps to minimize the profile or height of the power supplies. Moreover, the planar magnetic devices increase component reliability, reproducibility, and power density while minimizing the transformer leakage inductance. Planar magnetic devices are gaining more and more popularity in modern power supply design.
To achieve higher power, the resistance of the power magnetic device must be reduced, typically by either increasing the cross-section area of the electrical member forming the magnetic device windings, or by simply reducing the electrical path length of the device. In some cases, multiple windings or layers are connected in parallel to reduce the resistance.
As can be seen from FIGS. 1 and 2, there are two types of isolated power supply power structures, single ended 100 and double ended 200. In either case, there are two pieces of major power magnetic devices in most of isolated power supply 100 designs. Traditionally, two discrete surface mountable planar magnetic devices are used to realize the transformer 102 and the output filter inductor 104. Both of these power magnetic devices are mounted on an Insulated Metal Substrate (IMS) connected by the electric conductive copper trace(s) 106. For the double-ended power supply 200, there are additional traces 208, 210, 212 between the transformers 204, 206 and the inductor 214.
Having a separate transformer and inductor creates several disadvantages. First, it requires a longer electrical path for the termination and electrical connection. As shown, the connection goes from the transformer lead to the board, the conductive path on the board, and then from the board to the inductor lead. This requires additional material and thus increases cost. Further, this conductive path creates additional resistance. As resistance increases, so does the I2R power loss. The traditional conductive path also increases the complexity of co-planarity requirement of the magnetic devices terminations, requiring more connecting pins and/or headers. The traditional conductive path also takes more space. These shortcomings of the two discrete parts approach limit the power density of the power supplies using the planar magnetic devices. Therefore, what is needed in the art is a new method to integrate or combine the main planar power transformer and the output filter inductor.
Others have attempted to integrate the transformer and inductor. For example, U.S. Pat. No. 4,689,592 to Walker discloses a combined transformer and inductor. Walker discloses a single electromagnetic structure comprising a pair of assembled oppositely positioned pot cores with a flat magnetically permeable washer-like member inserted in the window area between the primary windings and secondary windings to form a combined transformer and the inductor. Unfortunately, the Walker approach creates several of its own problems. First, it increases the height and size of the single structure transformer core height. Next, it reduces the magnetizing inductance of the transformer due to a lower permeability gap introduced between the two core halves. The higher required inductance (i.e. the thicker magnetic short required), the lower the magnetizing inductance. Also, the number of primary winding turns required must be increased to compensate the reduced magnetizing inductance. This results in more I2R power loss. Further, the Walker technique makes the multi-layer planar winding board very difficult to manufacture due to the magnetic short between windings. Walker's technique also reduces window area for the winding structure due to the extra piece of the magnetic material. This results in more resistance for the windings and more power losses. Increased winding losses are also caused by fringing flux at the air gaps. Finally, the Walker technique makes the interleaving winding scheme to reduce the proximity effect more difficult.
Finally, there are examples of “open frame” power converters that rely upon a single board technique to create the complete converter including two or more magnetic devices. Examples include the Innoveta iQB series and the Synqor PowerQor series. In these converters, a single multilayer PWB forms the “mother board”, which contains windings for magnetic devices, conductive paths for the power train, and conductive paths to connect the control circuits together and to the power train. However, this technique requires a large, expensive multilayer PWB. The heat generated in the multilayer power windings is delivered to temperature sensitive control circuit components. Also, insertion of the cores around the mother board consumes valuable layout area on both sides of the PWB, which can result in a larger package size. Also, magnetic properties are difficult to test; the magnetic devices are an integral part of the converter product. Defects in the PWB windings can result in expensive scrap of the entire converter. Any changes on the transformer turns ratio due to the output voltage requirement require the multi-layer PWB to be modified, which results in high cost and high PWB inventory for same platform power supplies with different output voltages.